Such an annular mesh is the operative part of a bodily prosthesis that is commonly known as a stent. The purpose of the stent is to maintain a bodily lumen patent and, to do this, the mesh of the stent must resist the radially inward pressure of the bodily tissue that would otherwise close the bodily lumen.
As usage of annular mesh stents becomes ever more sophisticated, the demands for the annular mesh to be flexible, even while it resists radially inward pressure from bodily tissue, also increase. Stent designers have found it difficult to increase flexibility (in response to requirements for the longitudinal axis of the mesh to bend out of a straight line) while retaining adequate resistance to radially inward forces on the mesh.
Readers will readily appreciate that improvements in stent design could yield annular meshes that are interesting for application beyond bodily prostheses, whenever a combination of good resistance to radially inward force, and good bending flexibility, is required. The present invention may have applications beyond bodily prostheses and therefore the definition of the present invention refrains from limitation to stents.
Up to now, there have been two archetypal stent mesh designs, the first exhibiting a sequence of stenting rings each of which is a closed loop around the longitudinal axis. Adjacent stenting rings need to be connected so as to maintain a predetermined spacing between adjacent stenting rings along the length of the stent. Individual stenting rings have little or no capacity to bend when the longitudinal axis of the annular mesh is urged by external forces into a bent rather than a straight line, so the connectors between adjacent stenting rings carry most of the strain that allows such bending. Increasing the number of connectors increases the rigidity of the mesh, but an insufficient number of connectors can prejudice the integrity of the mesh. In consequence, many of the connectors evident in commercial stents are long and serpentine rather than short and straight. For examples of ring stents, see for example U.S. Pat. No. 6,770,089, US2002/0116051 and WO publications 2005/067816, 96/26689, 99/55253 and 03/055414.
The other characteristic form of a stent mesh is the helical stent, in which stenting struts precede as zig-zags around a spiral path from one end of the stent to the other. Connectors may be provided at spaced intervals, between successive turns of the spiral, for locational integrity of the mesh. A spiral form mesh has inherently more flexibility, and less resistance to radially inwardly directed forces, than is the case with a stack of closed stenting loops arranged transverse to the longitudinal axis of the annulus of the stent. For examples of helical stents, see for example, EP-A-1245203 and 870483, U.S. Pat. No. 6,053,940 and WO publications 2002/049544 and 01/01889.
Considering both the “ring stent” and “helical stent” categories, stenting loops advance around the circumference of the stent lumen as a zig-zag of stent struts that alternate with zones of inflection. Taking the line that is the bisector of the angle between two adjacent struts of a zig-zag loop, that bisector will likely lie parallel or near parallel to the longitudinal axis of the stent lumen, in any ring stent. Conversely, in any helical stent, that bisector will likely lie at an angle to the longitudinal axis that is larger as the helical pitch of the stenting loops gets larger.